An increasing number of LED driven luminaires are on the market today. Many of these systems are equipped with LED modules, which typically comprise a plurality of LEDs which are arranged on a base plate. In many applications the LED modules are fixed against a metal mounting surface of the luminaire. The performance and lifetime of these LED modules depend, for an important piece, on the interfaces between LED modules and luminaires because the luminaire casing will function as a heat sink.
A very common mechanical assembly structure for these LED modules utilizes screw connections which are screwed directly into the mounting surface via the housing of the LED-module, which is schematically illustrated in FIGS. 1a and 1b. In FIG. 1a and 1b a LED module 1 is illustrated in a mounted position on a mounting surface 2, i.e. the luminaire casing. The LED module 1 comprises a base plate 4, e.g. a printed card board, PCB, onto which a plurality of LEDs 3 are arranged. A plastic housing 5 covers the LED module 1. The LED module 1 further comprises a power connector (not shown). Here six screws 8 fixate the LED module 1 to the mounting surface 2 via the LED module housing 5. Different possible disadvantages and risks are connected with these kinds of screw connections. Firstly, the screws 8 have to be mounted with a defined torque because of the risk of stress cracking of the plastic parts in the housing 5. These torques differ from case to case and depend on many aspects like the type of screws, the hole dimensions for receiving screw holes in the mounting surface 2 of the luminaire when thread-forming screws are being used, the type of material of the mounting surface 2 etc. Another risk is that screws come loose in vibrating applications, e.g. street lighting applications, because of creepage effects in the plastic parts of the housing 5 and/or the low required initial screw torque. In this case LED modules will come loose, get overheated and break down. To overcome these risks customers have to take measures like using locking agent to lock the screws.
Further, a good thermal connection between the LED module 1 and the mounting surface 2 of the luminaire requires a well defined attachment between those two. Typically, a thermal interface material 7 is applied between the base plate 4 and the mounting surface 2 to avoid any air-gaps between the mounting surface and the LED module in order to get a lower and better defined thermal resistance. The performance of some thermal interface materials (e.g. thermal gap-pad materials) is very sensitive to how a LED module 1 is fixed at the mounting surface 2 in terms of the value of the pressing force F from the screws and the equability of the distribution of different pressing forces FR at the thermal interface, as illustrated in FIG. 1b. By using screws, very high peak forces are easily introduced. This can result in an unwanted bended bottom surface, as illustrated with arrows B in FIG. 1a, and also unwanted air gaps 9 in the critical thermal interface. This will have a negative impact on the thermal reliability and therefore the lifetime of the LED module 1.
To continue with reference to the prior art assembly of FIG. 1, by heating up (LED module 1 is switched on) and cooling down (LED module 1 is switched off) the different parts of the LED module 1 will move compared to each other because of their different thermal expansion behavior. By using screws 8 which apply locally high peak forces to the LED module 1, as described above, the different materials of the base plate 4 and the housing 5 cannot or can only move very little compared to each other. This can result in exceeding the maximum allowed stress level in the plastic housing 5 or that unwanted bending effects occur in these parts, as illustrated with the arrows indicated with B in FIG. 1a. 
LED modules are being sold with a long lifetime guarantee so they must withstand many thermal cycles meaning that the risk of material failures over time could be big especially in outdoor applications.